init Process
Init is the program which is run through the kernel at boot time. It is in charge of beginning all the normal processes which require running at boot time. These involve the getty processes which permit to login, FTP daemons, NFS daemons.
init is fast becoming the standard in the world of Unix to control the startup of software at boot time. This is due to the fact in which it is simpler to use and more flexible and powerful. It resides in /etc/rc.d.
/etc/rc.d/init.d
/etc/rc.d/rc0.d
/etc/rc.d/rc1.d
/etc/rc.d/rc2.d
/etc/rc.d/rc3.d
/etc/rc.d/rc4.d
/etc/rc.d/rc5.d
/etc/rc.d/rc6.d
The given directories hold scripts that are controlled through rc.sysinit process.
Note: All the given directories reside straightly in /etc in SUN UNIX and SCO UNIX.
Example
/etc/init.d, /etc/rc0.d
init.d holds a variety of scripts. Basically, one script for every service is needed to begin at boot time or when entering another run level. Services involve things like sendmail, nfs, httpd, etc. Services do not involve items like asset serial which must only be run once and then exited. Like services are found in rc.serial or rc.local.
Example
# /etc/rc.d/init.d/nfs start
# /etc/rc.d/init.d/nfs stop
init is the parent procedure of all the processes. The main role of this process is to create child processes from the script stored in the file /etc/inittab. This script has entries associated to init level through that all corresponding scripts get executed at startup. The init procedure is located in the /sbin/init.
Init is centrally configured by the /etc/inittab file. Within, the so-called "run levels" are described. It depends on the overall in /etc/inittab various scripts are beginning through init. These scripts are located in the /sbin/init.d folder.init which maintains the process of begining and shutting down.
/etc/inittab
The inittab file describes the processes which are begin at bootup and during normal operation. Format of the inittab file is as given below.
id:runlevel:action:process
Id - Id is unique identifier within the inittab file which identifies each entry with a limit of 1 to 4 characters.
Runs level - It Lists the run level for that the specified action should be taken.
Action - It describes the action to be taken.
Process - It specifies the process to be executed.
Action fields are as follows:
Respawn - This process will be restored whenever it terminates.
Wait -This process will be started once when the specified run level is entered and init will wait for its termination.
Boot -This process will be executed during the booting of the system. The run level field is disregard.
Initdefault -It specifies the run level entry, which should be entered after system boot. init will ask for a run level on the console if none exists. The process fields are disregard.
Sysinit - This process will be executed in during the booting of the system. sysinit will be executed before any boot or bootwait entries. The run level field is disregard.
Powerfail - This process will be executed whenever the power goes off. Init is commonly informed about this through a process which takes to a UPS that is connected to the system. The Init process does not wait for the process to complete.
the /etc/inittab file and init process work in the following manner
/etc/inittab
The init procedure would run entries with sysinit in the action field and entries within 3 in rstate field If init default is set to level 3.
Steps included in the init process are as follows:
1. This process reads the inittab file.
2. This process scans for other default run level by reading the initdefault entry.
3. This process executes the commands or scripts for entries which have sysinit in the action field.
4. This process executes the scripts for any entry which has a 3 in the rstate field. The Inittab resembles
Id: 5: initdefault:
Si:sysinit:/etc/rc.d/rc 0
10:0:wait:/etc/rc.d/rc 0
11:1:wait:/etc/rc.d/rc 1
12:2:wait:/etc/rc.d/rc 2
13:3:wait:/etc/rc.d/rc 3
14:4:wait:/etc/rc.d/rc 4
15:5:wait:/etc/rc.d/rc 1
16:6:wait:/etc/rc.d/rc 1